This invention is related to a protocol system wherein each station connected to a data bus of a data processing system must calculate its time to transmit turn during which it may begin transmitting data on the bus. This data processing equipment may be used for building automation, monitoring and control for fire and security management, airconditioning control, or the like.
Prior art data processing systems, for example those systems used for controlling or supervising the environment or fire and security points within a building, having a central panel for providing alarm or status indications of the various points within a building, originally provided a separate wire running from the panel to each supervised point. In today's large buildings, however, the cost of wiring in this manner can be prohibitive. Therefore, the prior art replaced these plurality of wires by a single transmission channel running from the central processing unit to the remote points being monitored.
The central processing unit is capable of scanning the status of the various remote points and controlling various operations at the remote point by addressing, in turn, each remote point and requiring each remote point to take some action. This action may be to report back its present status, such as temperature, pressure or humidity, or to perform some operation such as starting or stopping a fan, opening a damper or the like. By using one or more of these multiplexed transmission channels, the cost of installation of these monitoring systems is greatly reduced.
In such systems, the central processing unit typically consisted of a computer connected to one or more such channels running to the various remote points in a building. The size of the computer which was used in the central processing unit depends upon the number of remote points to be monitored and controlled. Because of flexibility and convenience, it has become necessary to configure such systems in such a manner that they can be expanded and can be interconnected for complex, large or multiple building facilities.
The present system comprises a plurality of remote stations each connected to a data bus. Each remote station may have a microprocessor or other type of computer connected through various circuits to the data bus and also connected to peripheral devices such as displays, supervised and control points, or the like.
Since the remote stations connected to the data bus may be physically separated by several miles, data communications between them is in serial form to permit transmission over common and inexpensive mediums or buses, such as phone lines and triaxial cables. Serial data transmission on a "party line" requires a protocol to prevent more than one device from transmitting at a time and to identify the transmitting device. This communication protocol is accomplished through a disclosed Transmission Protocol residing in each remote station.
A bus controller is not necessary since each remote station is capable of transmitting and receiving data independently of a controller. However, a bus monitor is utilized to make sure that the remote stations are transmitting in their proper turns. Functionally, each remote station continuously monitors all "party line" bus traffic. Based on present and past transmissions, each module "knows" its turn or time slot to transmit and transmits only at that time.